This invention relates to a resin-sealed, high-density semiconductor device having a large number of input/output pins (leads) and capable of operating at high speed.
A semiconductor device (or chip) having predetermined functions is generally sealed in a package by hermetic sealing or resin sealing methods for maintaining the stability of its characteristics, thermal or electrical for providing insulation between electrodes and for facilitating its of transportation and handling.
A package most often used for this purpose is a resin-sealed type. This is due to the fact that this type of package is superior in terms of its mass-production as well as from an economical viewpoint with respect to metal or ceramic packages and that its reliability and utility are better as a result of various improvements.
On the other hand, a glass-sealed type, mainly made of ceramics, has as good of heat dissipation properties and hermetic tightness as multi-layered ceramic packages (MLCP) and thus it is suitable for enveloping chips which handle high-speed signals or chips whose heat buildup is remarkable. Furthermore, it is cheaper to manufacture than an MLCP. In particular, it has an advantage that a large number of leads can be provided because its four sides can be used, and it is used increasingly to meet the requirements microcomputers of having thinner chips.
Lead frame materials currently used for these packages can be classified into an Fe-Ni group, a Cu group, an Fe group and so on. Of the above three groups, the Fe-Ni group material is preferred, depending on use, especially if high reliability is required for the IC's themselves.
The Fe-Ni group lead frames are ordinarily covered with a metal film to facilitate assembling such as chip bonding and wire bonding. Such a film is of gold or silver in the resin-sealed type of semiconductor device and aluminum in the glass-sealed type. These materials are used selectively taking into account the degree of reliability required for IC's, the manner in which Si chips are mounted, and the cost.
Next, the prior art will be described, taking the resin-sealed type as an example.
FIGS. 4 and 5 show sections of a conventional resin-sealed type of semiconductor device (IC package). As shown in the figures, gold or silver films 4 are formed on a die pad portion 2 and inner pad portions 3 of a lead frame 1 in a spot-like manner. This IC package is made by mounting a semiconductor element such as an Si chip 5 on the die pad portion 2 through the film 4, electrically connecting the chip 5 and the inner pad portions 3 of the lead frame by means of gold wires 6, and by forming a resin seal 7. To assure good bonding properties of the chip 5 and the wires 6, the metal film 4 is provided only on locations at which the wires and chip are bonded to the lead frame.
Numeral 8 designates a plating layer of tin or solder formed on the surface of an outer lead. This plating layer 8 is provided because if the lead frame is made of an Fe-Ni alloy or the like, its wettability with solder or the like is extremely bad and it is necessary to solder the frame to a printed circuit board or the like with high reliability.
Since a lead made of an Fe-Ni alloy is magnetic its self-inductance tends to be large during use of the device. Especially in the case of a high-density semiconductor device having a greater number of input and output pins, the lead portion inside the resin tends to be long and thus the self-inductance increases remarkably. This will hamper high-speed operation of the semiconductor device.